Amplifier circuit



y 1936- P. 0. FARNHAM 2,048,737

AMPLIFIER CIRCUIT Filed Sept. 25, 1931 E 7 5 Xm Circa/7'1 Patented July 28, 1936 f U: F

H UNITED ST TES 2,048,737 H AMPLIFIER-CIRCUIT i I Paul 0. Farnham, Bo'onton', N. J.,"assignor, "by

mesne assignments, to "*Radio Corporation of -America, New York, N'.--Y.', 'a corporation of Delaware Application September 23, 1931, Serial a e; w

11 Claims. (01178-44) This invention relates to audion amplifier cir .cuits, and more particularly to tuned amplifier circuits in which the gain and selectivity bear a desired relationship to the frequency to which a the am s V ,s g.

Itmay be stated that, in general, the most satisfactory tuned amplifier circuit will exhibit a constant gain and a constant selectivity-as it is tuned over the frequency band. In the following specification, selectivity is to be under stood as the ratio of transmission at the resonant frequency to the transmission at a constant frequency interval ofi resonance. In the simpler amplifier stages in which the couplingconsists of a transformer tuned by a variable condenser, the gain increases linearly with increasing fre- .qilency and the selectivity varies inversely with thefrequency. Unfortunately, the previous attempts to improve one of these characteristics of a transmission system including tuned am- ,plifier stages have reacted unfavorably upon the 1 other characteristic of the system. i e H An object of the present invention is to provide a tuned amplifier stage which reduces the variation of gain and ofselectivity with frequency, which reduction of variation may, in one special case, result in constant gain and constant selectivity over the entire frequency band. A further object is to provide anamplifier stage having a coupling system so designed and constructed that the impedances of circuit elements are varied automatically as the coupling system is tunedover the frequency band the variations being so related to frequency changes that the gain and the selectivityof theampli- ,fier both remain constant or approximately constant throughout the tuning range of the ampli- ,fier.

These and other objects of the invention will be apparent from the following specification when taken with the accompanying drawing, in which, 7 V a e I v 1 Fig. 1 is a schematic view of an amplifier including tuned coupling between successive tubes,

Fig. 2 is a circuit diagram of anequivalent Jnetwork, and figs. 3 and 4 are circuit mentsof the invention. e

In the drawing, the reference numerals l 2 identify successive tubes in an amplifier system, the plate circuit of tube l being coupled toth e control-grid or input circuit of tube 2 by means of a tuned coupling 3, The remaining circuit aconnections for energizing thetubes may be of any desired appropriate type. Through the amdiagrams of embodiplifier action of. tube 1, an alternating voltage rl=trg is established'in the plate fcircuit, identified in'Fig. 2'b'ythe'legend circuit I, and the coupli ig' 3 between circuit I and the grid circuit, II, of tube 2 sets up an alternating potential E2 betweenl'the control grid and cathode of tube 2.

'A'gen'e'ralized net work'quivalent to the coupled amplifier circuits is shown in Fig. 2, in whicheiifcuit '1 includes a voltagesource E1, in series "with aresistarice'Ri equal to the plate resistance 'of tube" land a coupling impedance Xs, QCir'cuitII includes, in series, the coupling reactan'cfxafa resistance R2 equal to there- 'sist'a'nce of the grid circuit oftube 2, a reactance X2TXni of the same type as the coupling reactianc'egand a 'v'ariable'reactanca X2, for tuning the circ'uit 'II.' That isjX'z is the sum of the reactances of a sign opposite to-that of the tuning elementfand, in one special case, Xm may be equal to X2, 1'. e., the total reactance of a sign oppositathat'of'the tuning reactance may" be embodied in the coupling reactance. is the lvolt'age develop'edacross'the tunable element of circuit II when the circuit is adjusted'for maximum E2 at the frequency of the desired signal. A mathematical study'o'f the Fig. 2 network will show that for constant gain and for cons'tantselectivity over the frequency band cer- "tain relationships between the magnitudes of the-"circuit elements must be satisfied. To sefrequency the following three equations must be s ti fies. i

Eff. fFQ

4 =K1, a constant (2) t a %=Kz, a constant (3) ft'hrough'out the frequencyban d while R1, the

la esi t nce of be must vary as; the square of the frequency; and

that, for inductive t n n s B2 mcust wa as: t ti m 0f the selectivity cure a gain and a selectivity independent of riable cathode resistance 6 is included in the circuit of tube l, and the control element of the resistance is mechanically connected to the movable element of condenser. 5, as indicated by bar v I. The total resistance, Byofthe'grid circuit. of tube 2 may be maintained substantially constant over the frequency band by the insertion in the circuit of a networlgof fixed impedances comprising a series resistance "8 shunted by a condenser 9. As the resonant frequency is increasedthe effect of resistance} upon the circuit resistance decreases automatically and the values of, resistance 8 and condenser v9 are so chosen-that the total resistance of the grid circuit'does not vary appreciably with frequency.

The resistance 8 mayhave a value ofabout 120 ohmswith a condenser-9 of about 0.007 microfarad'." I

One-possible arrangement of an inductively P tuned coupling system is shown, in Fig. 4, as

including a tuned plate'inductance l0 coupled, by condenserILto'the grid circuit whichincludes a variable resistance l2 and capacity l3 between the control grid and cathode of tube "2. The adjustable elements of tuning inductance l0 and the resistance-l2 are'mechanically linked, as indicated by bar l4,',to secure such variation of the resistance with tuning that thetotal resistance of the grid circuit variesinversely with the'square of the frequency. The plate resistance of tube l remains constant throughoutthe tuningrange. 1 'It will be apparent that a. 'pluralityof stages embodyingthe invention may be cascaded in the usual manner, and that the particular circuits herein described and illustrated are illustrative of two particular networks based upon the generalized network shown inFig. 2. Otheradapta- 'tions of that general network will occur to those familiar withthe design of electrical" circuits.

While the foregoing explanation has dealt with the securing of constant gain and'constant selectivity over the-frequency band, it is to be understood that the invention is not restricted to methods or circuit arrangements which maintain both the gain and the selectivity absolutely constant. The invention provides a general method for thecontrol of the frequency variation of gain and of selectivity and, for various reasons, it may be desirable to construct amplifier circuits in"which the usual frequency variation of one or both of these characteristicsis altered-but is not completely eliminated.

-' Iclaim: v

1 In a' transmission system, a pair 'of circuits serially arranged in the path of signal transmission and coupled'by a capacitive reactance, an

adjustable resistance in one of 'said'circuits, an

adjustable inductive reactance in oneof said circuits for tuning the same: over a band of frequencies; and means for simultaneously adjusting said adjustable tuning reactance and said adjustable resistance, said adjustable resistance beinga series element in the second of said cir- -2. 'In-an amplifier; the combination with a pair automatically increasingthe resistance of the cuits while} maintainingfthe-resistarice of the of vacuum tubes, an inductively tuned output circuit for the first tube, capacitive means coupling the input circuit of said second tube to the output circuit of said first tube, an adjustable resistance included in said input circuit, and means automatically decreasing the efiective magnitude of said resistance assaid output circuit is tuned to higher frequencies.

3. In a transmission system, the combination with a circuit in which a signal voltage is developed, a second circuit inductively coupled to said first circuit and including an adjustable condenser for tuning said second circuit over a band of frequencies, of resistive impedance in said second circuit, and means comprising a network of fixed impedancesfor automatically maintaining the effective value, of said resistive impedance substantially constant as the resonant frequency of said second circuit is increased.

4. Theinvention as set forth in claim 3, in 20 combination with resistance in said first circuit, and means automatically increasing the said resistance as the resonant frequency of saidsec-- ond circuit is increased. l V 1 5. In an alternating current transmission system, the combination with two circuits, reactance coupling said circuits, andreactive means'for tuning one of said circuits over aband'offre quencies,g of resistance in one of said circuits,

and'means operative automatically with adjust- 3;

ment of said reactive tuning means to vary said resistance to substantially match thejimpedance of both said circuits throughout the tuning range and thereby maintain the gain and selectivity of said coupled circuits substantially independent 3 of the frequency to which said system is tuned.-

6l A vacuum tube amplifier Of:;the typeineluding two vacuum tubes, and a condenser tuned transformer coupling said tubes, of" means primary circuit of said transformer and automatically reducing the increase in resistance of the transformer secondary circuit as the transformer is tuned to higher frequencies.

'7. In the operation of a tuned amplifier of the type in which two tubes are cascaded by coupling the plate circuit of one tube to the input circuit of the second tube by a coupling reactance of one type and a tuning reactance of the opposite type included in one of saidcircuits,--the method of controlling both the'amplifier gain and the amplifier selectivity as afunction of the resonant frequency, which comprises automatically varying the resistance of one of said cirother circuit substantially'constant as theam'pli- 'fier is tuned over its freq'uency'bandz- 8. The invention as set forthin claim 7, wherein the rates of variation of the resistances of said tively coupled to the input'circuit of the-succeeding tube, and one of saidcircuitsincludes an adjustable condenser for tuningthe amplifier circuits are so'related to the resonant frequency {3b of an amplifier of'theitype v a q ency-band, the method which comf prises automatically maintaining the resistance of the input circuit substantially constant over the frequency band while varying-the plate circuit resistance as the square of the frequency of the amplifier is tuned over'its frequency bandi 10. In the operation of a tuned amplifier including two tubes cascaded by a capacitive coupling system, the method of reducing the frequency variation of gain and of selectivity which comprises tuning said coupling system by adjusting the magnitude of an inductive reactance therein, and simultaneously varying the resistance of the input circuit of the second tube inversely as the square of the resonant frequency.

11. In an alternating current transmission system, the combination with two circuits, inductive impedance coupling said circuits, and

capacitive means for tuning one of said circuits over a band of frequencies, of an impedance in one of said circuits, and means operative automatically with adjustments of said capacitive means to vary one of said impedances to substantially match the impedance of both said circuits throughout the tuning range and thereby maintain the gain and the selectivity of said coupled circuits substantially independent of the adjustment of said capacitive means.

PAUL O. FARNHAM. 

